1. Field of the Invention
The invention relates to the field of lighting, more specifically, in a preferred embodiment, the invention relates to a Light Emitting Diode (LED) signal with a single optical element.
2. Description of the Related Art
Common incandescent lamp signals, typically have a single light source (a point source), a parabolic reflector and a distribution cover. The energy and maintenance efficiency of LED signals has resulted in their widespread adoption in place of incandescent lamps.
Previous LED signals have difficulty presenting a uniform display aspect. Individual points of light (from individual LEDs) or shadows from portions of the display aspect which are not completely covered by the LED light distribution pattern detract from uniformity (intensity homogeneity) of the prior LED signals.
As shown in FIG. 1, typical previous LED signals use a plurality of LEDs 20, a collimating/positive element/lens 5 (collimating element), and a distribution/spreading cover/window/lens 40 (distribution cover). Light emitted in a conical pattern from each of the LEDs 20 is directed towards the collimating element 5, which is often in the form of a Fresnel lens or other form of positive lens having a single, specific, focal length. That is, generally parallel light rays impinging on a first side of the lens are focused by the lens onto a single point on the other side of the lens. The collimating element 5 collects and refracts the light into a generally uniform, parallel direction (i.e. collimates the LED light output). The collimated light passes to the distribution cover 40 which is required to create a specific light distribution, spreading the narrow collimated beam of light into a desired pattern optimized for the anticipated position(s) of the viewer(s), for example pedestrians and or vehicle operators in the case of traffic signals.
Previous solutions to the intensity homogeneity problems include purposely locating the LED""s at a location out of focus with respect to a focal length of a collimating positive lens. However, designing the signal to be xe2x80x9cout of focusxe2x80x9d causes a light output design loss. Alternatively, the LEDs may be located at an increased distance from the optical elements in a pattern where each of their light outputs overlap. The increased distance between the optical element and the LEDs allows the light emitted by the LEDs to fully cover the optical elements. Increasing the distance between the LEDs and the optical elements requires a deeper housing which may frustrate use of the LED signal in retrofit applications to existing incandescent housings and adds materials costs compared to a shallower housing.
In order to meet existing signal standards and/or allow retrofitting into signals originally manufactured for use with incandescent light sources, LED signals mimic the front housing diameter and depth restrictions of the prior incandescent signals. To allow use of large diameter Fresnel lenses as the collimating element 5 without requiring an overly deep housing, previous signals use a collimating element configured with outward facing features 15. Because dirt will accumulate on the outward facing features of an exposed collimating element and degrade light output, a distribution cover with a generally smooth outer surface was required on the previous solutions to seal the collimating element from the environment even if creation of a light distribution pattern other than the narrow collimated light was not important.
Each additional optical element introdeces a light transmission loss and adds extra matrerials and assembly costs. It is an objective of the present invention to solve these and other problems that will become apparent to one skilled in the art upon review of this specification.
Light transmission losses, materials and assembly costs are minimized by using only a single optical element. One or more LEDs are clustered to create a light emission zone which illuminates a single optical element that forms the desired final light distribution pattern specific to the anticipated application of the signal. The aggregate light from the light emission zone is transformed by the single optical element into a desired final display aspect. The single optical element uses Fresnel elements in regions close to the center of the optical element and total internal reflection elements towards the outer periphery of the optical element. Both the Fresnel and the total internal reflection elements are formed on the inner face of the single optical element. The outer surface of the single optical element is designed so that features, for example diffusion patterns, thereon on which dirt accumulation may occur are minimized or eliminated.